1. Field of the Invention
The present invention relates generally to the problem of the controlled deposition of organic layers onto substrates, and more particularly to the production of spatially patterned layers and multilayers for organic electronic devices and other applications, as well as devices produced by use of such methods.
The field of organic semiconductor optoelectronics is attracting increasing attention for many application sectors including displays, lighting, electronics, photodetection, solar energy conversion, and communications. These and other potential application areas require the ability to fabricate devices such as light emitting diodes, solar cells, photodiodes, transistors, optical amplifiers, and lasers. A strong attraction lies in the potential to form physically flexible, plastic devices. In all cases a low cost, reliable fabrication method is required for these devices to be able to compete with the entrenched silicon and other inorganic semiconductor device technologies. The ideal organic layer deposition technique should include the following: compatibility with high throughput (e.g. reel to reel) processing; the ability to create multilayer and multi-element structures; and compatibility with atmospheric processing conditions (i.e. not requiring costly vacuum environments). The manufacture of high quality devices also requires deposition techniques that can produce uniform, substantially defect-free layers of micron and sub-micron thicknesses and control their location on the substrate with high spatial resolution.
Organic semiconductors can be classified as small molecules, dendrimers, or polymers. Small molecules have a small number of atoms and a precise chemical structure and are often difficult to process from solution to form uniform thin films. Typical examples include N,N′-diphenyl-N,N-bis(3-methylphenyl)1-1′-biphenyl-4,4′-diamine (TPD) and 8-tris-hydroxyquinoline aluminum (Alq3). They are generally processed via traditional vacuum deposition techniques. Polymers on the other hand have a large number of atoms typically arranged with a repeating chain-like structure and with a dispersity in chain lengths (and hence molecular weights) within a given sample that equates with an imprecision in chemical structure. They are typically readily processed from solution or in the melt. Examples including poly(9,9-dioctylfluorene) (PFO) and poly(3-hexyl thiophene) (P3HT). Solution processing has been widely adopted for polymer device fabrication, including spin-coating, blade-coating, ink jet printing, and gravure printing. The third class, namely dendrimers, has the chemical precision of small molecules (precise molecular weight) but can be readily soluble and have substantially higher molecular weight. They have an architecture comprising a core from which a series of branches (dendrons) emanate. Solution processing is also viable with dendrimers.
In the fabrication of organic devices additional layers may also be needed including for example conducting layers for electrodes, insulating dielectric layers, light control layers, and so on. The invention described herein equally addresses the deposition of semiconducting, conducting, insulating, and otherwise functional organic layers.
As already noted, many solution deposition techniques have been developed for use with organic materials, including inkjet printing, doctor blade coating, and gravure printing techniques for multicolor light emitting diode and polymer transistor fabrication. However, these solution-based techniques are largely unsuitable for multilayer deposition since the solvent used for one layer can often also dissolve or partially dissolve, swell, or otherwise disrupt previously deposited layers. Finding so-called orthogonal solvents (orthogonal solvents are solvents that behave as good solvents for the layer to be deposited but that do not dissolve or unfavorably swell or disrupt the underlying layers) for the organic materials to be deposited and/or re-designing the materials specifically to enable the use of orthogonal solvents is at best time consuming and at worst impossible. Using “precursor routes” to form layers of insoluble organics has also not proven especially useful, due to the difficulty of achieving full conversion from the precursor and avoiding unwanted side reactions during the thermal/photo conversion process. High temperatures and/or acidic environments incompatible with the manufacture of certain devices may be needed to approach full conversion for example of sulphonium precursors to polyphenylenevinylene derivatives. Another issue for solution processing concerns the need to spatially define the location at which a particular material is to be deposited. The substrate may need to be pre-structured using lithography or focused beam etching to define areas into/onto which the material can be selectively deposited. This adds complexity and cost.
Organic Vapor Phase deposition can be used for deposition of multiple layers of high quality but is essentially limited to small molecules and conventionally requires a vacuum environment that is problematic for high throughput manufacture. Thermal transfer and laser assisted thermal deposition are also under development as means to deposit patterned organic film structures, but they generally require the development of specifically modified materials rather than being straightforwardly compatible with existing organic semiconductors.
Micro-molding-transfer (μMT) and micro contact printing (μCP) techniques have also attracted considerable attention for use in depositing organic layers. Both techniques conventionally use a polydimethylsiloxane (PDMS) elastomer stamp as a carrier for the material to be deposited. In μMT a liquid precursor fills a micro channel patterned in the surface of a PDMS block. The precursor is then partially cured and subsequently brought into contact with the substrate. The partially cured film attaches onto the substrate and the flexible mold is removed leaving the film behind. This technique has been used to produce multilayer 3D microfluidic structures and polymer based lightwave integrated circuits from optical (non-conjugated) polymers. However, this technique involves a chemical curing process that is not available for most existing organic materials of interest, and the thickness of the films produced is typically in the micron range, which is too large for the requirement of many electronics applications. The morphology of the resulting layer is also often non-ideal due to a lack of control over the curing process. This in turn results in the formation of non-uniform structures that are undesirable within the context of a device fabrication process.
In contrast to μMT, the μCP technique transfers material to a substrate through contact with the protruding surface of a PDMS stamp. The μCP technique has been used to transfer self assembled molecules (e.g. thiols) to metal substrates to act as resists for lithography, to transfer proteins to biochips, and to transfer water soluble conducting polymers such as Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) (hereafter PEDOT:PSS) to device substrates. The potential to transfer a thin uniform layer of a functional polymer is an important advantage of μCP over μMT. However, this method has been developed for water-soluble materials while many of the organic materials of interest require the use of organic solvents rather than water.
The PDMS stamp is key to micro contact printing techniques. First, the PDMS stamp at room temperature is sufficiently soft that it is easy to achieve conformal contact between the stamp and a substrate, a situation that promotes transfer. Second, its surface is hydrophobic, but can be converted temporarily into a hydrophilic surface after oxygen or air plasma treatment, due to the formation of a very thin (about 2 nm), rigid SiOx layer on the surface of the stamp. This very thin layer is, however, not stable, and the surface reverts to its hydrophobic form over time. A film deposited on the surface of the PDMS stamp therefore gradually reduces its adhesion to the stamp.
Theoretically, any type of film deposited on a PDMS stamp can be transferred to any substrate using μCP, but in practice this is not the case. In order to successfully (uniformly) transfer the film from a PDMS stamp to the substrate, conformal contact with good adhesion has to be established between the film and the substrate. For example van der Waals forces between a gold film on the stamp and gold on the substrate, or static electric forces between a biomolecule on a stamp and its antibody on the substrate, very much help to promote successful transfer.
PEDOT:PSS, a water-soluble polymer, is one of the very few polymers that has been efficiently transferred to a substrate using μCP. A water rich environment helps the PEDOT:PSS to swell and deform, and helps the formation of static electric and van der Waals forces between the PEDOT:PSS and the substrate during the conformal contact stage. The μCP of typical non-water soluble polymer semiconductors using a PDMS stamp to transfer them to a substrate is in contrast difficult, since the polymer chains are relatively rigid and stiff, adversely affecting conformal contact and adhesion. Furthermore, conventional μCP usually produces an inverted U or an M cross-section for the deposited layer element due to the non-uniform film that forms when the stamp is coated with polymer solution.
The layer thickness, uniformity and quality (especially with respect to an absence of pinholes and voids) are essential parameters for devices containing functional organic films, such as light emitting diodes, photodiodes, and transistors. Very thin, uniform, and high quality films are essential for organic electronics in order to ensure the best optoelectronics performance and durability and it is for this reason that μCP has not been widely adopted as a preferred manufacturing technique.